Clutch
There are so many different clutch designs so I will cover just a few that are used in Motorcycle and ATV engines. Listed below are ones that Banshee & RZ 350’s use and I will keep adding information as I have time. Almost all of the references that I will be covering are Banshee/RZ based as that is the engine we are racing at this moment. As TDR builds more engine designs, we will most likely use the same components.
Multiple Plate: The multiple plate theory is that with more friction area the clutch has more friction area to allow it to grab better. This added grabbing force is needed to hold the clutch from slipping under full load. This load happens on initial launch and most important at top RPM when the motor is making full horsepower.
On a street bike this is not needed but on a race engine the ability to have the hardest acceleration at the start of the race is a great advantage. Especially when drag Racing. Stock clutches are designed for overall performance and reliability with the OEM horsepower level. As racers and performance minded riders build more horsepower the need changes. Banshee & RZ stock clutches are 7 friction plates and 6 steels. I will break down all the important components and explain them here.
Stack Height: This is the combined height of all the steel and friction plates measured together. Why is this important? The stack height is extremely important on drag racing applications as the height changes by wear it gets smaller. This smaller height will affect the amount of pressure that the springs put on the assembly. There is a specific set height that you will start with, and it will have an exact amount of spring tension pressure. As the stack height gets thinner the pressure changes as the spring rate changes as it compressed length changes. If you want the launch to be the same every time you need to have that same clutch spring pressure every time. If you are ever at a Motorcycle Pro Stock Drag you will see them changing the clutch every time out. Not that it is worn out but is has worn and the launch will change if it is left in. They are taking the worn clutch and measuring the stack height and adding other used or new friction plates to get that exact measurement for the next clutch change or race. This way you have much more control of the launch and 60 foot times.
Spring Tension: Spring tension should be measured at the exact length that it is at the rested or compressed distance it is when your clutch is fully engaged, meaning clutch lever out. Measuring each spring at that distance and multiplying by the number of springs your clutch has gives the exact amount of pressure compressing your clutch. This gives you a reference point that you can always use as a base pressure. I install the clutch pack using 3 springs and measure the distance that the spring is compressed. I then test a spring rate at that distance. This is how you know how much pressure that 1 spring has. Multiply that by 6 and you have the full pressure rating that is pushing on the clutch friction plates. Here is where it starts getting more complicated. With the stock 7 friction plates how much horsepower will it take before the clutch breaks loose? Once you have that information you can start calculating how much gripping power each friction plate can hold.
Friction Plate service limits: You should always measure the friction plate and refer to each manufacturers minimum thickness.
Steel Plates: RZ/Banshee steel plates are 1.0mm thick. We sell steel clutch plates in different thicknesses. The reason is to increase clutch pressure to aid in slipping we add an extra 1.2 mm steel plate to increase stack height. Another way is to replace a stock 1.2 mm thick steel plate with one that is 1.78mm or 2.29mm. Another trick is to groove the steel plate to create a sharp edge for the fiber plates to grab.
Race Track Tips: As mentioned above the adding of an extra steel plate creates more spring pressure and making a groove with a die grinder will help a slipping clutch and get you thru the next race. Go to our YouTube channel to see videos of different clutch tips and detailed explanations of all the different clutch packages we sell. https://www.youtube.com/watch?v=0iE5HjSQ14c&t=1s